The present invention relates to electron guns, and particularly to a cathode ray tube having an arrangement of electron gun electrodes and current limiting arc-suppression resistors. The arc-suppression resistors, in the case of an electrical arc, will operate as a voltage divider to limit the potential in the gaps between electrodes to less than the full ultor potential, thereby limiting the arc current and preventing damaging cascading arcs.
Cascading arcs are defined as a succession of rapidly initiating arcs between electrodes in high field regions of the electron gun which permit a sufficiently high arc current to pass between electrodes of the electron gun and to subsequently damage the electron gun elements or the associated circuitry.
A conventional cathode-ray tube, for example, a color television picture tube, consists of an evacuated envelope having a neck portion, a faceplate and a funnel portion therebetween. An electron gun is disposed in the neck portion of the envelope, and a tricolor emitting phosphor screen is disposed on the interior surface of the faceplate. A shadow mask is located between the electron gun and the screen, in spaced relation to the screen. The electron gun comprises a plurality of electrodes for focusing and accelerating three electron beams toward the phosphor screen. Typically, several high voltage and low voltage electrodes are serially arranged along the electron beam paths to facilitate the focusing and accelerating of the electron beams. The high voltage electrodes typically operate at an ultor potential of about 30 kilovolts, and the low voltage electrodes typically operate at about 8 to 10 kilovolts or less; however, in some electron guns, an intermediate potential of about 12 kilovolts and a low potential of about 8 kilovolts or less are utilized. A conductive coating having an effective resistance of about 100 ohms is disposed on the interior surface of the funnel portion of the envelope. The interior conductive coating operates at ultor potential. Bulb spacers mounted on the electron gun electrode nearest the phosphor screen contact the interior conductive coating to provide ultor potential to the electron gun. An exterior conductive coating, electrically isolated from the interior conductive coating is provided on the outside of the funnel to facilitate grounding of the envelope. The interior and exterior conductive coatings on the funnel also serve as a large capacitor which filters the high voltage.
The large voltage difference established between the high voltage and low voltage electrodes in the electron gun creates a possibility of arcing between the electrodes. The possibility of arcing is increased by irregular electrode surfaces, foreign matter in the interelectrode gaps and by misalignment or improper spacing between electrodes. When an arc occurs, the high voltage filter capacitor will, within a few microseconds or less, discharge its stored charge.
Because the instantaneous peak arc currents can approach hundreds or even thousands of amperes in magnitude, great destruction can be caused by such arcs. The external electron gun circuitry can be damaged by transient currents and voltages induced into the associated receiver circuitry. The gun electrodes can be burned or eroded to the point of inoperability, and electrode material may be sputtered onto adjacent surfaces resulting in the creation of leakage paths between tube elements.
In order to reduce tube arcing and to minimize the damage caused thereby, it is common to design cathode-ray tubes with maximum electrode spacings, to minimize field gradients and to incorporate arc suppression systems into the tube.
U.S. Pat. No. 2,829,292 issued to De Vere Krause on Apr. 1, 1958 describes one of the earliest attempts to limit arc currents to nondestructive levels. In the De Vere Krause structure, a high-resistance internal coating having a resistance of about 1 megohm is provided over a portion of the neck of the envelope. The high-resistance coating is disposed between the end of the electron gun nearest the screen and the conventional anode coating on the interior surface of the funnel. When an arc occurs between adjacent electrodes, the high-resistance coating limits the arc current to a fraction of an ampere. Unfortunately, the conductive metal film released during the getter flash frequently shunts the high-resistance coating so that the arc suppression structure described in the De Vere Krause patent is unreliable. Variations of the De Vere Krause structure using resistive coatings or discrete resistors between the anode coating on the funnel and the electron gun have been proposed by other workers in the art.
U.S. Pat. No. 4,101,803 issued to Retsky et al. on July 18, 1978 discloses an arc suppression structure utilizing both a resistive anti-static neck coating extending between the electron gun and the conventional anode coating, and a discrete resistor in the antenna getter support wire, i.e., a parallel resistive network. Both the anti-static coating and the resistive surface portion of the discrete resistor are formed of a resistive frit. A drawback of the resistive frit arc-suppression system is the disclosure that the resistance decreases by several orders of magnitude following the vacuum bake of the tube. This means that the value of the resistance cannot be accurately determined until a point in the tube processing when the resistance cannot be altered. Additionally, Retsky et al. admit that tubes have not yet been constructed in which no arcing whatsoever occurs. In such tubes, if an arc occurs across the parallel resistor network, a high probability exists that the arc will cascade between the adjacent electrodes and damage the gun circuitry. Thus, there exists a need for an arc-suppression system which will minimize the damage to the gun elements and the associated circuitry by limiting the arc current.
Such a structure is disclosed in U.S. Pat. No. 4,345,185 issued to Y. Kobori on Aug. 17, 1982 and discussed by Y. Kobori et al. in their paper entitled, "A Novel Arc-Suppression Technique For Cathode Ray Tubes", presented at the IEEE Chicago Spring Conference on Consumer Electronics, June 19, 1980. In the Kobori structure, a ceramic resistor is connected between the G3 and G5 high voltage electrodes (typically 30 kV) and another resistor is connected between the low voltage G4 electrode and the stem lead attached thereto. Such a structure is shown in FIGS. 3 and 4. When arcing takes place between adjacent electrodes, the arc current may flow either from the G5 through G3 to the G2 or to the G4. As will be described in detail hereinafter, a problem occurs when the initial arc and the resulting plasma generated thereby results in additional arcs, e.g., cascading arcs, between the other electrodes of the electron gun across the interelectrode gaps A, B and C of FIG. 4. In this case, the full arc current flows through the gun electrodes into the receiver causing possible damage to the electron gun components and to the associated gun circuitry.
Therefore, an arc-suppression system must be able to protect the electron gun not only from the effects of individual arcs but from the effects of a multiple arc. A multiple arc is herein defined as a succession of rapidly occurring arcs resulting from an initial arc. The arc-suppression system should be one in which the arc current is properly limited to prevent damage to the gun elements and to the gun circuitry.